This invention relates to a composite gas separation membrane and to a process for preparing such a membrane.
Specifically, the invention relates to a composite polysulfone-zeolite membrane for separating hydrogen from a hydrogen/carbon dioxide mixture, and to a process for preparing the membrane.
In some industrial processes such as the production of hydrogen for fuel cells or hydrogenation, or pharmaceutical processes such as enzymatic catalyzed reactions there is a need for the removal or recovery of hydrogen from a mixture containing hydrogen and carbon dioxide without a phase change. (G. Hxc3xa4rtel et al, Separation of a CO2/H2 gas mixture under high pressure with polyethylene terephthalate membranes, J. Membrane Sci. 113 (1996), 115-120 and G. Hxc3xa4rtel et al, Permselectivity of a PA6 membrane for the separation of a compressed CO2/H2 gas mixture at elevated pressures, J. Membrane Sci. 162 (1999) 1-8). Although membrane technology has gained acceptance in some gas separation applications, the separation of hydrogen from carbon dioxide is difficult to accomplish using membranes derived from traditional polymers. This is because hydrogen and carbon dioxide both have high permeability coefficients compared with other gases such as methane, nitrogen and oxygen. Typically, the hydrogen/carbon dioxide separation factor [ISF] is rather low for many glassy polymers, e.g., in the vicinity of 2-3 (G. C. Kapantaidakis et al, Gas permeation through PSF-PI miscible blend membranes, J. Membrane. Sci., 110 (1996) 239-247; A. Alentiev et al, Gas permeation properties of phenylene oxide polymers, J. Membrane Sci., 138 (1998) 99-107, and Y. Shindo et al, Calculation methods for multicomponent gas separation by permeation, Sep. Sci. Tech., 20(5 and 6) (1985) 445-459). Similar ISF values are observed when a mixture of hydrogen and carbon dioxide is used. Only a limited number of studies can be found on the application of membranes to the separation of hydrogen and carbon dioxide gas mixtures. For example, Hxc3xa4rtel et al (G. Hxc3xa4rtel et al, Separation of a CO2/H2 gas mixture under high pressure with polyethylene terephthalate membranes, J. Membrane Sci. 113 (1996), 115-120 used a polyethylene terephthalate membrane to separate a 50/50 volume mixture of hydrogen and carbon dioxide. The separation was done under a high feed pressure (70 bar) with a 4 bar differential pressure and they were able to achieve a selectivity of around 4 at steady-state under the given conditions.
On the other hand, the advantage of composite membranes comprising glassy polymersand molecular sieves are evidenced in many studies such as T. M. Gxc3xcr, Permselectivity of zeolite filled polysulfone gas separation membranes, J. Membrane Sci. 93 (1994) 283-298; C. Joly et al, Sol-gel polyimide-silica composite membrane; gas transport properties, J. Membrane Sci. 130 (1997) 63-74; M. G. Sxc3xcer et al, Gas permeation characteristics of polymer-zeolite mixed matrix membranes, J. Membrane Sci. 91, (1994) 77-86; M. Smaihi et al, Gas separation properties of hybrid imide-siloxane copolymers with various silica contents, J. Membrane Sci. 161 (1999) 157-170 and C. M. Zimmerman et al, Tailoring mixed matrix composite membranes for gas separations, J. Membrane Sci. 137 (1997) 145-154. Zeolite and carbon molecular sieves have shown favorable effects for this application. Ideally, hydrogen passes through the well-defined channels of a proper molecular sieve such as zeolite 3A, while the permeation of carbon dioxide is hindered because of its larger molecular size. From the data of Lennard-Jones (L. M. Robeson, Correleation of separation factor versus permeability for polymeric membranes, J. Membrane Sci. 62 (1991) 165-185), the kinetic diameters of the two gases are H2=2.89 xc3x85 and CO2=3.3 xc3x85. Since the diameter of H2 is less than that of the well-defined and uniform zeolite 3 xc3x85 pores, and the diameter of CO2 is larger, an increase in the selectivity is expected for H2/CO2 if the gas mixture diffuses through the zeolite pores. For an enhanced separation to work in practice, all or most of the hydrogen molecules must pass through the zeolite channels rather than the voids between the zeolite and the polymer matrix. Such voids often occur due to the poor adhesion of zeolite particles to the polymer matrix.
There have been a number of attempts to incorporate zeolite into polymer matrices in order to improve membrane separation (M. G. Sxc3xcer et al, Gas permeation characteristics of polymer-zeolite mixed matrix membranes, J. Membrane Sci. 91, (1994) 77-86 and T. M. Gxc3xcr, Permselectivity of zeolite filled polysulfone gas separation membranes, J. Membrane Sci. 93 (1994) 283-298). As described in D. W. Breck, Zeolite Molecular Sieves, John Wiley, New York. 1974; R. Szostak, Molecular Sieves; Principles of Synthesis and Identification, Blackie Academic and Professional, London, Second edition 1998 and M. E. Davis, The quest for extra-large pore, crystalline molecular sieves, Chem. Eur. J.3 (11)(1997) 1745-1750, zeolites have different structural types with pore sizes ranging from small (3 xc3x85) to extra-large (15 xc3x85). The addition of zeolite into a continuous polymer phase induces a microporous cavity and channelling system of a defined size in the zeolite-polymer composite membrane. Significant differences in measured permeability and calculated selectivity values demonstrate the importance of the type and percentage of zeolite. Permeabilities and selectivities are enhanced at high zeolite loadings in the polymer matrix with zeolites 13X and 4A for H2/N2 and CO2/N2 gas separations (M. G. Sxc3xcer et al, Gas permeation characteristics of polymer-zeolite mixed matrix membranes, J. Membrane Sci. 91, (1994) 77-86), but there is no performance increase for H2/CO2. Gurkan et al (T. Gurkan et al, A new composite membrane for selective transport of gases, Proc. 6th Int. Symp. Synthetic Membranes in Science and Industry, Tubingen, Germany, August 1989) reported the separation of O2/N2 and H2/N2 gas pairs using a zeolite 13X-polysulfone membrane made by extrusion. A substantial increase in permselectivities was observed when compared with pure polysulfone.
One problem associated with zeolitexe2x80x94glassy-polymer composite membranes is the formation of voids around the zeolite particles due to poor adhesion of the polymer to the external zeolite surface (see I. E. J greater than Vankelecom et al, Incorporation of Zeolites in Polyimide Membranes, J. Phys. Chem., 99(35), (1995) 13187-13192).
An object of the present invention is to provide a solution to the above-identified problem in the form of a functionalized glassy polymer for separating gas pairs, i.e. gases containing molecules of two different types.
Another object of the invention is to provide polysulfone-zeolite composite membrane for separating a gas pair such as hydrogen/carbon dioxide by exploiting the well defined and substantially uniform zeolite pores for selective diffusion of one member of the gas pair, namely hydrogen in the H2/CO2 pair.
In a specific embodiment of the invention, the use of zeolite 3A (pore size of about 3 angstroms) as a molecular sieve has been found to provide results differing from those in some previous reports where the zeolite pore size permits passage of both gases. For example, zeolite 5A for CO2/CH4 separation showed no change in selectivity (J. M. Duval et al in Adsorbent filled membranes for gas separation. Part 1 (Improvement of the gas separation properties of polymeric membranes by incorporation of microporous adsorbents, J. Membrane Sci. 80 (1993) 189-198), 70% silicalite filled PDMS membranes used for O2/N2 separation showed a modest selectivity increase (M. Jia, et al, Molecular sieving effect of the zeolite-filled silicone rubber membranes in gas permeation, J. Membrane Sci. 57 (1991) 289-296). Zeolite 13X used for separating several different gases showed either no pronounced effect (T. M. Gur, supra) or some enhancement in permselectivity (M. G. Suer et al, supra).
In order to enhance the membrane selectivity, the inventors provide a method for covalently attaching zeolite particules to the polymer chain, thereby reducing or eliminating the presence of void spaces between the two phases. This is achieved using an aminofunctional methoxysilane as a coupling agent to bind the zeolite particles to an aldehyde modified polysulfone matrix. It is believed that the aldehyde functional group of the polymer reacts with the amino group of the coupling agent which itself binds to the zeolite surface by reaction of silyl ether with zeolite-OH as shown in the reaction scheme of FIG. 1. Membrane preparation conditions and the factors affecting gas permeation and permselectivity are determined using a zeolite 3A molecular sieve composite membrane.